Curt, Steve Stone used to always ask:
"when was the last time you saw an Olympic sprinter dive to the finish line?"
I think that no one should ever slide into first base, except in the rare instance of avoiding a tag when the throw is off line. However, the Olympic sprinter comment doesn't seem to be valid. A sprinter has to touch the tape with his chest. This prevents him from merely reaching forward with his had to touch it earlier, which I feel certain would work.